A polyolefin-based porous film excels in mechanical properties, chemical resistance, electric properties and the like, and is used for packaging material use, medical use, electric material use and the like. Among them, in the electric material use, a polyolefin-based porous film is used to design a safer secondary battery by using a function as a separator for separating a positive electrode from a negative electrode and making only ions permeate therethrough, and a shutdown function for preventing a risk of a battery reaction to be went out of control due to a temperature increase inside of the battery (function for closing pores at a temperature lower than a thermal runaway temperature).
A method for producing a polyolefin-based porous film is classified broadly into two methods, a wet method for forming pores in an extraction step, and a dry method for forming pores in a stretching step (also called stretching-opening method).
Examples of the former producing method include a method described in Patent Document 1. Patent Document 1 discloses a producing technology for improving mechanical strength (tension strength, modulus of elasticity in tension) by making birefringence of a wet film composed of three components, which is formed by melt kneading and extruding a polyethylene to which a plasticizer (DOP) and silicate fine particles are added, and then, by extracting the plasticizer and the silicate fine particles in an extraction tank to form pores, be 1.0×10−3 or more.
On the other hand, examples of the latter producing method include methods described in Patent Documents 2 to 5. Patent Document 2 discloses a producing method of a polyethylene porous film, and discloses a basic technology in which a raw fabric before providing pores therein (hereinafter, also referred to as “non-porous raw fabric”) is MD cold stretched, and then, is MD heat stretched. Patent Document 3 discloses a relationship between a blow-up ratio and various physical properties in an inflation molding using a circular die. Patent Document 4 discloses a technology for adding less than 10 mass % of ethylene-propylene rubber (EPR) and ethylene-propylene-diene rubber (EPDM) so as to improve mechanical strength. Patent Document 5 discloses a three-layered (PP/PE/PP) laminate porous film made of a polypropylene (PP) and a polyethylene (PE), and also discloses making birefringence of a polyethylene raw fabric before stretching be within a range of 2×10−2 to 4×10−2 and various physical properties including a pore surface area of the obtained laminate porous film. Patent Document 6 discloses that, as a technology for restoring a decrease in a film width generated by MD heat stretching, a three-layered (PP/PE/PP) laminate porous film is formed as a porous film, and a composition of an isotactic polypropylene and a random polypropylene is used as a material for the porous film. Patent Document 7 discloses, as a producing method of a biaxially-stretched porous film using a dry method, a technique for transverse stretching a porous film obtained by a known vertical uniaxially stretching while relaxing in a vertical direction in a hot state.